Oxides of transition metals are known as inorganic materials having characteristic absorption properties in the visible light range. According to the absorption principles of these oxides, when oxygen is coordinated with a transition metal, the d orbitals of the transition metal are divided into some energy levels. The electron transition energy in the orbital is 1-3 eV. Since this electron transition energy overlaps the light energy in the visible light range, the oxide causes characteristic coloring in the visible light range. A composite oxide composed of two or more transition metals is considered to absorb light in the visible light range based on the same principles.
Industrial applications of colored films made of such oxides include glass products comprising a transparent glass plate covered with a colored film to lower the transmittance of visible light for giving automobile and house windows a privacy protection capability, and glass products which block solar radiation to cut off heat radiation and ultraviolet radiation. Since these glass products are used in house, building, and automobile windows, they require colored films of highly durable properties including wear resistance, chemical resistance, etc.
Such colored films are fabricated by vacuum evaporation, sputtering, etc. These dry processes need a highly expensive vacuum system. Many automobile window glasses are bent to meet design requirements. Such glass plates as industrial products do not lend themselves to being applied to substrates in such dry processes.
One simple process for fabricating a thin film inexpensively without the need for an expensive system is a thermal decomposition process in which a material such as a metal salt is dissolved in a solvent to prepare a solution, the solution is applied to a substrate, and heat-treated at a high temperature to produce a metal oxide film. While this process is capable of fabricating an inexpensive colored film, the colored film produced does not have sufficient wear resistance, chemical resistance, etc. from practical standpoints.
There have been proposed a coating solution for manufacturing a colored film, the coating solution being a combination of the solution of the metal salt according to the thermal decomposition process and a metal alkoxide added thereto that is vitrified by a sol-gel process, and a process for manufacturing a colored film (see J. Non-Crystalline Solids, 82, (1986), pages 378-390).
Specifically, a film of an oxide of Si, for example, which is highly wear-resistant and chemical resistant, is prepared by the sol-gel process, and a transition metal which is present as a coloring component in the oxide film is converted into an oxide by the thermal decomposition process. A coating solution is prepared by dissolving nitrates or sulfates of several transition metals as a coloring component into a solvent according to the intended purpose, and an Si alkoxide, for example, is added to the coating solution and mixed therewith. The mixed solution of the metal salt solution and the hydrolyzed alkoxide is coated on a substrate. When the coated solution is heated, a porous gel having a molecular structure of Si --O--Si is produced. When the coated solution is further heated, the transition metal ions present in the porous gel are crystallized with the heat and separated into an oxide which serves as a coloring component. The gel is further baked into a rigid silica film. As a result, a silica glass film colored with the transition metal oxide is produced as a colored film which is resistant to wear and chemicals.
Japanese laid-open patent publication No. 9-169546 discloses the following details: This publication states, as a problem with the above proposal, that when a silicon alkoxide or the like other than the coloring component is added to make the film sufficiently durable, the absorbance is lowered, and the film needs to be thickened in order to achieve a necessary reduction in the transmittance.
The publication discloses in claim 1 thereof a coating solution for forming an oxide film, including at least one metal salt selected from the group consisting of Co, Cr, Mn, Fe, Ni, Cu, Zn and a lanthanoid, and an ethylene glycol oligomer.
The publication discloses in claim 3 thereof that an alkoxide or chelate of at least one metal selected from the group consisting of Si, Ti, and Zr is contained in the above coating solution.
The publication discloses in example 14 thereof a coating solution comprising Co, Mn, and SiOR, in example 15 thereof a coating solution comprising Co, Fe, and ZrOR, and in example 16 thereof a coating solution comprising Co, Ni, and TiOR.
Each of the films obtained from those examples has a brownish transparent film appearance similar to a half-silvered mirror, and has a reflectance ranging from 20% to 35%. The publication states that the haze value, adhesion, and chemical resistance of the films obtained are good.
The publication discloses in example 21 thereof a coating solution comprising Cu, Mn, and SiOR, in example 22 thereof a coating solution comprising Cu, Mn, Co, and SiOR, and in example 24 thereof a coating solution comprising Cu, Mn, Co, Cr, and SiOR.
Each of the films obtained from those examples has a black transparent film appearance, and has a reflectance of 8%. The publication states that the haze value, adhesion, and chemical resistance of the films obtained are good.
Glass is manufactured according to the sol-gel process as follows: A metal alkoxide is hydrolyzed and then polymerized into a three-dimensional network of metal atoms and oxygen atoms. The reaction is continued to turn the network into a porous gel, which is then heated to produce glass and a polycrystalline oxide.
One of the features of the sol-gel process is that it is capable of synthesizing glass at a low temperature. Particularly, silica glass synthesized at a low temperature from an Si alkoxide is widely used as hard coating films on surfaces of plastic products, etc.
Several colored films produced according to the sol-gel process have been proposed in the art.
For example, Japanese laid-open patent publication No. 5-9406 proposes a colored glass gel thin film containing a metal alkoxide, a condensed polymer of a metal alkoxide, a dye, an alcohol solvent, and a compatible dispersant. The dye has a particle diameter ranging from 300 to 20000 nm, and may comprise a metal oxide as an inorganic pigment.
According to Japanese laid-open patent publication No. 5-9406, the particle diameter of the dye needs to be adjusted to keep the colored glass gel thin film transparent.
It may be possible to form a colored film directly using an alkoxide of a transition metal. While alkoxides of Si, Ti, Al, Zr, etc. are inexpensive and can be handled relatively easily, alkoxides of transition metals are expensive and mostly cannot be handled with ease. Therefore, forming a colored film directly using an alkoxide of a transition metal would not be a universal and general process for forming a colored film.
In order to increase the wear resistance and chemical resistance in the above proposal according to J. Non-Crystalline Solids, it is necessary to increase the amount of an added metal alkoxide. If the amount of an added metal alkoxide were simply increased, the coloring of the thin film by the metal oxide would be very weakened, the transmittance of the thin film would be increased, and the color tone of the thin film would become yellowish. As a result, a color tone essentially represented by the coloring component would not be achieved.
In each of examples 14, 15, 16 and examples 21, 22, 24 according to Japanese laid-open patent publication No. 9-169546, an alkoxide of at least one metal selected from the group consisting of Si, Ti, and Zr is contained in a coating solution for forming an oxide film, which comprises a metal salt and an ethylene glycol oligomer.
In each of examples 14, 15, 16, a film having a brownish transparent film appearance similar to a half-silvered mirror is produced. In each of examples 21, 22, 24, a film having a black transparent film appearance is produced.
Japanese laid-open patent publication No. 9-169546 discloses nothing about reasons for the different film appearances and detailed film forming conditions.
It is therefore an object of the present invention to provide a method of manufacturing a glass plate with a light-transmissive colored film using a coating solution which is prepared by adding a metal alkoxide as a solution containing metal salts directed to a thermal decomposition process, particularly with the light-transmissive colored film having a coloring effect due to a metal oxide which will not be impaired even if the amount of the added metal alkoxide is increased, and being highly resistant to wear and chemicals.